A construction machine such as a hydraulic excavator is an example of a work machine having an actuator for driving a drive object. Such a construction machine burns fuel (gasoline, light oil or the like) in an engine to drive a hydraulic pump. Pressurized fluid thus produced drives a hydraulic actuator (a hydraulic motor, a hydraulic cylinder or the like). Hydraulic actuators are small-sized and light in weight and can produce significant power; therefore, they are widely employed as actuators for construction machines.
On the other hand, hybrid construction machines have recently been proposed which is intended to achieve an improvement in energy efficiency and energy saving, compared with the conventional construction machines using only hydraulic actuators. The hybrid construction machines use electric machinery (an electric motor) and an electrical storage device (a battery, an electric double layer capacitor or the like), which supplies and receives electricity to and from the electric machinery. In comparison with the hydraulic actuator, the electric machinery (the electric actuator) has, in terms of energy, excellent features such as (1) excellent energy efficiency and (2) regeneration of kinetic energy as electric energy during braking (release as heat in the case of a hydraulic actuator).
One of the hybrid work machines of this type is the construction machine as below (JP,A 124381). A hydraulic motor and electric machinery serving as actuators for swingably driving a swing structure and an operating device which outputs, through one operation, operating signals for simultaneously actuating the hydraulic motor and the electric machinery are mounted on the construction machine. In addition, the swing structure is driven by the total torque of the hydraulic motor and the electric machinery. This construction machine is such that the electric machinery is directly connected to the hydraulic motor. Output torque for each of the actuators (the hydraulic motor and the electric machinery) is determined according to the operation amount of the operating device (the swing control lever). For example, at the time of deceleration (braking), the electric machinery regenerates the kinetic energy of the swing structure and stores it as electric energy in the electric storage device. This technology combines the electric machinery and the hydraulic motor as the actuators for driving the swing structure. Therefore, even the operators who are familiar with the conventional hydraulic-actuator-drive construction machine can operate the operating device without a sense of discomfort. In addition, the technology intends to achieve energy saving with the simplified and easily realized configuration.